The Japanese government, which had shown “e-Japan strategy” and “e-Japan priority plans” previously, has now presented a goal to formation of a high-grade information communication network society and concrete policies required to achieve the goal with priority. One of the subject matters to achieve the great plans is fusion of broadcasting and communications. If the broadcasting that has accumulated a variety of abundant programs and contents and computer networks that have been improved rapidly in convenience and serviceability are fused into one, new network services will be born. Such expectations have been increasingly built up in recent years.
A streaming technique is one of the techniques that have been most expected to realize such broadcasting services on the existing networks. In case of a streaming service, it is required to reproduce frames at specified times, respectively, so that the data including the one to be reproduced several seconds later are held in an application buffer. If this buffer does not store data enough to be reproduced consecutively, the reproduction is suspended until a certain amount of data are accumulated in the buffer. Consequently, the quality of broadcasting services comes to be affected significantly by delays and jittering of the communication, as well as frame losses. Preventing such frame losses has thus been considered to be most important in those broadcasting services.
The current IP (Internet Protocol) network prevents such frame losses through the retransmission control by the TCP (Transport Control Protocol), which is an upper layer protocol. However, the retransmission control might cause communication delays to increase. This is why the retransmission control cannot apply to such services as broadcasting services that do not allow significant delays. And under such circumstances, the UDP (User Datagram Protocol) is used as an upper layer protocol for transferring broadcasting service data. The UDP does not have any function to prevent frame losses as described above, however. A protection switching function is effective to duplicate the communication route, thereby preventing such frame losses. Particularly, the 1+1 protection switching function that enables a copy of each frame to be sent from a sender apparatus to a plurality of routes, those frames and their copies to be selected at and transferred from the receiver apparatus is the most effective means to prevent the frame losses as described above.
In case of the 1+1 protection switching function, the sender apparatus adds a sequence number to each frame and copies the frame, then sends out those frames and their copies into a plurality of communication routes. A frame and its copy are given a same sequence number. On the other hand, the receiver apparatus checks the sequence number of each frame received from the plurality of communication routes and selects and transfers only normal frames.
In case of the technique disclosed in JP-A No. 2006-100900, the sender apparatus sends VoIP (Voice over IP) frames to a plurality of routes. And the receiver apparatus stores the frames received from the plurality of routes in a frame buffer of which addresses are assigned so as to correspond to the sequence numbers of those frames, respectively. The receiver apparatus then reads those frames from the frame buffer in the order of sequence numbers, thereby transferring the frames in the order of sequence numbers. Consequently, frame missing and frame disordering are prevented without requiring any complicated processings such as frame sorting in the buffer.